Various types of magneto ignition systems are known; the present invention is specifically directed to a magneto system in which a primary of a magneto ignition coil is magnetically coupled to a field rotating in synchronism with rotation of the engine. The secondary of the coil is connected to a spark plug. The ignition instant itself is determined by a control circuit, likewise coupled to the primary, which first controls a transistor to be conductive so that current can flow through the magneto ignition coil primary to store magnetic energy therein, and then is controlled, suddenly, to cut off, the sudden interruption of current flow inducing a voltage in the secondary which triggers the spark in the spark plug. In such systems, a positive voltage half-wave in the primary first controls the ignition transistor to be conductive. At about the peak value of the voltage half-wave, a threshold switch is triggered which, in turn, controls the ignition transistor to cut off. Adjustment of the ignition timing with respect to the magneto is obtained by generating auxiliary voltage half-waves, first a small voltage half-wave and subsequently a greater positive half-wave. In low-speed ranges of the machine, only the larger voltage half-wave will cause the threshold switch to respond. Since, as the speed increases, the voltage in the primary also rises, the threshold switch will respond at high speed ranges already when the earlier, and smaller voltage half-wave occurs, since under high-speed conditions, the smaller voltage half-wave will have reached the threshold level. Thus, at a certain critical speed, the ignition timing is advanced by a fixed amount in the direction of spark advance.
The efficiency of operation of internal combustion engines which operate within wide speed ranges is poor in an intermediate speed range if the ignition timing at that intermediate speed range is not matched to the actual engine speed. Changing ignition timing in jumps by a fixed values is undesirable when the engine is not operating either at a low idle speed or at a high speed, but is apt to be operated over wide ranges of speed. The generation of a separate half-wave control voltage to trigger the threshold switch at high-speed operation requires additional apparatus in the primary circuit, which is costly and undesirable from the manufacturing point of view.